Summary:
The Isibonelo Colliery is located in the Highveld Coalfield where four coal seams are present. These consist of, from bottom, the underdeveloped No 2 Seam, sequentially to the No 5 Seam at the top. Only the No 4 Seam is declared as Coal Resources and Coal Reserves.
No faulting was detected during exploration drilling or mining activities. A sill identified on the aeromagnetic survey, as well as in boreholes, is situated above the No 4 Seam and has little effect on the coal seam. Three thin dolerite dykes were intersected during mining, but with little effect on the mining or coal seam.
The coal seams in the Highveld Coalfield are generally described as flat lying to gently undulating, with a regional dip to the south. The depth of the coal seams increases in a southerly direction. The Highveld coalfield has been affected by numerous dolerite dykes and sills related to the Drakensberg Formation flood basalts. Within the vicinity of the dolerite, the usual structural complications and devolatilization of the coal occurs.
At surface, the soils are described as sandy loams to sandy clay loams. Towards the south, the soil type becomes a dark grey sandy clay, due to the weathering of the underlying dolerite sill. Any outcrops or sub-crops are dominated by sedimentary rocks of the Ecca Group (Vryheid Formation).
The depth of weathering ranges from less than 1.0 m to almost 30 m in a few isolated holes located within the low-lying areas. The weathered zone predominantly comprises a grey-brown sandstone, with minor clay rich intercalations. The impact that the deeply weathered areas may have on the stability of the highwall within the current opencast areas has been reviewed. The resistivity data derived from the Dighem survey was compared to the point data from the drill holes in order to produce a continuous, calibrated surface representing the interface of soft and hard interburden. This surface was included in the geological model to allow a more precise estimation of volumes of the various materials.
In order to model the weathering surfaces more accurately, Isibonelo has created a short term geological model which is updated at site on a regular basis. The model is updated with information obtained from face mapping, geophysical logs, and drill holes.
Pre-Karoo Basement
The glacially sculpted pre-Karoo basement comprises metasedimentary and metavolcanic rocks of the Paleoproterozoic Waterberg Group and post-Waterberg intrusives (felsites and granites). Due to the majority of the drill holes being drilled to just below the No 2 Seam, very few holes (2.5%) intersected basement at Isibonelo. Where the basement was intersected, it has been described as a light red, fine-grained granite.
Ecca Group
Within the Ecca Group, the Vryheid formation comprises zones of alternating conglomerates, sandstone and mudstones within which the coal seams are located. The stratigraphy of the Vryheid Formation may be divided into three main sequences at Isibonelo; a basal No 2 Seam Sequence, overlain by the No 4 Seam Sequence, which in turn is overlain by the No 5 Seam Sequence.
No 1 Seam Sequence
The Number 1 Seam (No 1 Seam) is only sporadically developed at Isibonelo. It has not been included in the geological model and does not form part of the resource base.
No 2 Seam Sequence
The No 2 Seam sequence includes the succession from the top of basement to the top of the No 2 Seam and incorporates the rocks of the Dwyka Group, as well as the overlying No 1 and No 2 coal seams.
At Isibonelo, the rocks of the Vryheid formation lie directly on top of the Dwyka Group, comprising both diamictites and varved shales. The clastic interval below the No 2 Seam comprises a fine to course-grained sandstone, with the occasional granulestone/gritstone at the base of the succession.
This upward fining sequence from the top of the Dwyka to the base of the No 2 Seam, where intersected, is in the region of 25 m thick, increasing to over 40 m towards the east (South Pit).
The No 2 Seam, where developed, is on average 1.25 m thick, and comprises a coal and carbonaceous mudstone rich interval. As such the No 2 Seam is not considered economic and does not form part of the resource base.
No 4 Seam Sequence – Target Seam
The No 4 Seam Sequence comprises the succession from the immediate roof of the No 2 Seam to the top of the No 4 Seam. The immediate floor of the No 4 Seam predominantly comprises a fine to medium grained sandstone that is sometimes bioturbated and often contains siltstone laminations.
The No 4 Seam is well developed across the entire Isibonelo licence area and is the sole economic target. The seam is on average 5.6 m thick, ranging from 4.2 m to almost 7 m in places. The seam is divided into three zones, based on the lithological variation of the seam. The zones are named from the base up as the Number 4 Seam Lower Zone (S4Z1), Number 4 Seam the Middle Zone (S4Z2) and the Number 4 Seam Upper Zone (S4Z3). The Lower Zone is the thickest and best quality of the three zones (21.7% average ash content); the Middle Zone is generally a poor quality coaly shale with multiple mudstone partings (45% average ash content), and the Upper Zone is a dull-lustrous coal with shaley lenses and occasional bright bands (31% average ash content). Currently the full No 4 Seam (S4Z1+S4Z2+S4Z3) is mined.
No 5 Seam Sequence
The No 5 Seam Sequence comprises the succession from the immediate roof of the No 4 Seam to the top of the No 5 Seam.
The No 5 Seam’s extent is limited by the depth of weathering, as the seam ranges from 15 to 23 m below surface. The seam is on average 1.8 m thick, and as per the No 4 Seam, is divided into three distinct lithological zones, namely the Number 5 Seam Base (S5B), Number 5 Seam Parting (S5P) and Number 5 Seam Top (S5T) from the base up. The S5B is the lower most zone; it averages 0.78 m in thickness and comprises a dull-lustrous coal, with bright bands, and an occasional in-seam carbonaceous shale parting. The S5P is a finely laminated carbonaceous shale, and has an average thickness of 0.64 m. The S5T is on average 0.36 m thick and comprises a relatively bright coal with an abundance of calcite on the cleats of the coal. The full No 5 Seam has an average ash content of 42%.